Automatic shifting transmissions use fluid operated friction torque transmitting assemblies, such as clutches and brakes, to control the speed ratio between the transmission input shaft and the transmission output shaft. The speed ratio is changed by interchanging at least one of the torque transmitting assemblies for another of the torque transmitting assemblies. It is common practice to control the pressure rise in the oncoming torque transmitting assembly with a trim valve.
The trim is generally a downstream regulator valve having a variable bias pressure imposed thereon. The bias pressure is, with the advent of electronic controls, established by a variable bleed solenoid valve. The solenoid valve has an output pressure that varies in accordance with a control voltage. An electronic device, such as a preprogrammed digital computer, establishes the desired voltage input to the solenoid valve. The output from the solenoid is a variable hydraulic bias pressure that is proportional to the voltage. The output pressure of the trim valve is proportional to the variable hydraulic pressure. The feed pressure to the trim valve is generally main control pressure which is much greater than the maximum variable hydraulic bias pressure.
The hydraulic bias pressure at the trim valve has a range of zero to approximately 70 psi. The trim valve output pressure is required to have a range of approximately zero to approximately 250 psi depending on the torque capacity required by the oncoming torque transmitting assembly. The trim valve therefore has an overall gain of approximately 3.5 which is determined by dividing the maximum output pressure by the maximum bias pressure. As can be appreciated from the above gain value, every 1.0 psi change of the bias pressure results in approximately a 3.5 psi change in the output pressure.
Since the presently available trim valves use a linear gain valve, it is extremely difficult to provide a smooth ratio interchange when it is necessary to control the oncoming torque transmitting assembly to be fully engaged at a trim valve output pressure of 50 psi or less. This means that the bias pressure is approximately 14.3 psi which is established in the lower 21 percent (%) of the output range of the solenoid valve. Accurate pressure control under these conditions requires either a very close tolerance on the valve members or extremely accurate electronic control and feedback or both resulting in higher manufacturing and inspection costs.